1. Field of the Invention
The present invention relates to a motor-driven injection molding machine using a drive motor (servomotor) serving as a drive source.
2. Description of the Relevant Art
Generally, a motor-driven injection molding machine using a servomotor as a drive source potentially involves malfunction or breakage of the servomotor due to overheat arising under certain molding conditions or under a certain environment, since the servomotor generates heat according to load current. Therefore, the motor-driven injection molding machine usually employs measures for protection of the servomotor. Specifically, a heat generation value of the servomotor is detected. When the detected heat generation value reaches a preset stopping point, the value is interpreted as indicating overload, and the servomotor is forcibly stopped to thereby protect the servomotor.
For an injection molding machine, stoppage of a servomotor during molding means interruption of a molding process, which may have an adverse effect on the molding process once resumed. Therefore, interruption of a molding process must be avoided to the extent possible. In order to meet the end, Japanese Patent Application Laid-Open (kokai) No. 11(1999)-235743 proposes a method for checking preset operating conditions. According to the proposed method, the injection molding machine is operated on a trial basis, and heat generation value (estimated heat generation value) of a servomotor is calculated from load current flowing through the servomotor. On the basis of the result of the calculation, it is judged whether or not the servomotor operates in an overloaded unstable region. When the servomotor operates in the overloaded unstable region, the preset operating conditions are reviewed so as to impart appropriate allowance thereto, to thereby avoid interruption of operation (molding process) during actual molding.
The above-mentioned conventional method for detecting an overload state involves the following problems.
First, since heat generation value of a servomotor is detected indirectly from load current flowing through the servomotor, the detected heat generation value may differ from the actual heat generation value, which is affected by a change in ambient atmosphere (room temperature) and other factors, so that the conventional method is neither reliable nor stable.
Second, since the preset operating conditions are reviewed at the stage of trial operation performed before regular molding operation so as to impart appropriate allowance thereto, the injection molding machine may fail to operate under optimum operating conditions, and cannot flexibly cope with the actual molding state, resulting in impaired molding quality or productivity.
An object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of realizing a dual protection function through direct monitoring and indirect monitoring, thereby enhancing reliability and stability in protecting drive motors.
Another object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of flexibly coping with the actual molding state and of always conducting molding under optimum molding conditions to thereby avoid impairment in molding quality and productivity.
To achieve the above objects, the present invention provides a method for controlling a motor-driven injection molding machine in which heat generation of a drive section including a drive motor is monitored, and when the drive section enters an overload state, the drive motor is stopped. In the method, during molding, heat generation value (overload factor) of the drive motor is estimated from load current flowing through the drive motor. When the estimated heat generation value (overload factor) reaches a preset stopping point or when a thermostat attached to the drive section outputs a signal indicating an overload state, the drive motor is stopped.